For certain applications it is necessary to provide single phase A.C. from three phase lines. For example, a mainframe computer may be designed to operate on three phase power because of its increased reliability, while peripheral equipment to be used in connection with the computer would typically be designed to operate on single phase. In such a case, the single phase may be obtained from two of the three phase lines.
However, in such an arrangement, a problem arises if the single phase to which the peripheral equipment is connected is interrupted. Thus, due to the provision of voltage regulation, the computer will continue to operate satisfactorily on two phases, but the peripheral equipment may be lost altogether. Since the loss of peripheral equipment could render the system useless, it is important to ensure that such equipment continues to operate when a single phase of the three phase power source is lost.